Here goes nothing. First post on the sub so I hope I don't disgrace myself.

1. I think that like most things, it's subjective and very dependent on genre. It's probably close to neutral overall, but so much depends on why you're watching that particular anime. Originality and plot twists are fantastic, but sometimes you don't want to be on the edge of your seat, trying to figure out the next twist in the tale. Every now and again, some people might want to switch the brain off and enjoy some escapism with a decently executed harem comedy, or something.

A lack of predictability has its place, no doubt about it. But it's also worth considering that without 'predictable' shows that follow and codify tropes and tread the well-worth path of cliche, we wouldn't appreciate (or even have?) 'original' work, which is original in the sense that it deviates from the norm. If everything is original, nothing is - kind of.

1. I vote for execution being slightly more important. There are so many intangibles, but two shows that I watched recently were Toradora! and Shin Sekai Yori. The former is considered a top-tier example of a typical romance anime, whilst the latter is feted for its original premise and world. But for me, Toradora was definitely the superior anime as it was so well-written and executed, even if I knew exactly what was going to happen by the end - although admittedly it wasn't entirely predictable. Whilst I enjoyed Shin Sekai Yori a lot, I thought the presentation and pacing could've been better, particularly in the first half, and the artistic direction was...questionable, for me at any rate. It really is quite subjective and experience-dependent.

Those are just two examples; more extreme would be Serial Experiments Lain, which is very unpredictable but isn't necessarily that popular...it's almost too far outside the curve.

1. You could argue that nothing is truly original anymore! The first thing to sprang to mind as an example was One Piece, maybe HxH too - shows that have very original worlds, characters and several twists, but also stick to long term shonen archetypes and tropes for the backbone of their narratives. It's obvious what's going to happen (Gon and Luffy will, eventually, reach their goals), but the ride is still fun.